This invention relates to a light beam scanning apparatus. More particularly, it relates to a light beam scanning apparatus wherein a subject to be scanned is scanned with a light beam in a two-dimensional manner, for example, a stimulable phosphor sheet having a radiation image stored therein is exposed to stimulating rays to perform reading or a photographic light-sensitive material is exposed to a light beam to record an image therein, and which includes means for detecting any positioning error including the position of the leading or trailing edge and/or the inclination of the stimulable phosphor sheet or photographic material prior to reading or recording.
One of typical prior art methods for obtaining radiation images as picture images is a radiation system using a radiation film having an emulsion layer comprising a silver halide photo-sensitive material in combination with a sensitizing paper. As a substitute for the radiation system, attention is recently drawn to a radiation image recording and reproducing system using a stimulable phosphor.
Certain phosphors, when exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, electron beams and ultraviolet rays, store part of the energy the radiation possesses. When the phosphor is subsequently exposed to a stimulating ray such as visible light, it produces stimulated emission corresponding to the stored energy. The phosphor exhibiting such properties is known as a stimulable phosphor.
There is proposed a system for recording and reproducing radiation image information using the stimulable phosphor. In such a system, the radiation image information of an object such as a human body is first stored in a sheet having a layer of stimulable phosphor (referred to as "stimulable phosphor sheet" or simply "phosphor sheet", hereinafter). The sheet is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The emitted light is detected by a photoelectric detector to obtain image signals as electrical signals. The image signals are processed to provide a radiation image of the object ready for diagnosis. Reference is made to U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, and 4,387,428 and Japanese Patent Application Kokai No. 56-11395.
The system of this type converts the radiation image information stored in the stimulable phosphor sheet into electrical signals, appropriately processes the signals, and outputs the processed signals as a visible image on a recording material such as photographic light-sensitive material or on a display such as a cathode ray tube (CRT), providing a radiation image having improved adaptability for image reading by observation or diagnostic adaptability.
The radiation image information recording/reproducing system includes an image reading unit for reading a radiation image stored in a stimulable phosphor sheet. The following is illustrative of the procedure for the reading unit to read the radiation image.
The stimulable phosphor sheet having a radiation image stored therein is scanned with a light beam such as a laser beam in a two-dimensional manner whereupon the resulting stimulated emission is sequentially detected in time series by a photoelectric detector such as a photomultiplier to obtain image signals. The two-dimensional scanning of light beam is accomplished, for example, by applying a linearly deflected light beam to the stimulable phosphor sheet to perform main scanning, and carrying the sheet in a direction substantially perpendicular to the main scanning direction by means of two pairs of synchronously driven rollers while clamping the sheet therebetween, thereby performing auxiliary scanning or sub-scanning. See Japanese Patent Application Kokai No. 62-94068 or U.S. Ser. No. 920,337.
Thereafter, the image information obtained as above is fed to an image recording unit. The image is recorded by modulating a laser beam in accordance with the image information derived from the stimulable phosphor sheet, and applying the modulated beam to a recording material in the form of a photographic light-sensitive material to thereby record the image in the photographic material. The photographic material having the image newly recorded therein is subjected to development and then collected in archives such that it may be accessible for medical diagnosis as required.
The image reading unit has the likelihood that the stimulable phosphor sheet is misaligned during the process of main scanning by the applied light beam. If the misaligned phosphor sheet is carried for sub-scanning by the two pairs of synchronously driven rollers, the positions for starting and/or finishing read scanning might be off the correct positions, or read scanning might be carried out on the phosphor sheet positioned at an angle to the scanning direction. Then the spot of a light beam on the phosphor sheet is off the desired spot. If scanning with laser beam is continued on the phosphor sheet inclined with respect to the two pairs of rollers, the resulting image information would be inaccurate. Differently stated, no correct radiation image information is derived from the wrongly positioned stimulable phosphor sheet. This can undesirably invite an error in medical diagnosis when the object is a patient.
In the image recording unit, an image can be recorded in a photographic light-sensitive material by exposing the photographic material to a modulated laser beam in a deflected fashion to effect main scanning, and carrying the sheet in a direction substantially perpendicular to the main scanning direction by means of two pairs of synchronously driven rollers while clamping the sheet therebetween to effect sub-scanning, thereby accomplishing two-dimensional scanning.
The image recording unit also has the likelihood that the photographic material is misaligned during the process of main scanning by the application of a laser beam modulated in accordance with the radiation image information stored as electrical signals. If the misaligned photographic material is carried for sub-scanning by the two pairs of synchronously driven rollers, then the spot of the laser beam is off the desired spot on the photographic material. The thus recorded image is inaccurate as including a loss of information. If the photographic material having such inaccurate image information recorded therein is used for medical diagnosis, there is the likelihood of incurring a wrong analysis.